The gas lift technique has been widely used in crude oil production in order to efficiently lift a heavy oil, which is slightly lighter in weight than seawater, from a submarine oil field having a low natural flow pressure. In this technique, a gas having a small and relatively negligible mass is injected into a liquid inside a riser pipe to reduce the pressure of a liquid column inside the pipe by an amount corresponding to the volume ratio of the gas, and thereby a pushing force by a pressure of the seawater outside the riser pipe or a reservoir pressure is generated at the lower end portion of the riser pipe. It is confirmed that the bubble lift (including air lift or gas lift) technique is effectively usable even for a mineral or the like having a high specific gravity when the mineral is formed into slurry by being finely crushed and mixed with seawater or the like.
As an example of this type, as described in Japanese Patent Application Publication No. 2005-291171, a bubble jet type air lift pump or the like has been proposed to aim at an efficient apparatus which is capable of removing sediments such as earth, sand and sludge settled and compressed on the bottom of sea, a lake, a clamor a liquid storage tank, and achieves high liquid pumping performance. And a bubble jet type air lift pump including an air lift riser (riser pipe) through the inside of which water and air rise, and a bubble jet generator provided at a bottom portion of the air lift riser and configured to jet out water mixed with bubbles are proposed.
In the case of slurry, earth, sand, sludge, or the like (hereinafter collectively referred to as slurry) having a much higher specific gravity than the surrounding liquid (seawater or water), unless a considerable volume of bubbles is injected, the average specific gravity of the fluid mixture of the slurry and the gas in the pipe cannot be reduced to or below the specific gravity of the surrounding liquid. In this case, an upward flow cannot be generated inside the riser pipe, and consequently the pushing force due to a pressure difference between the inside and the outside cannot be generated at the lower end portion of the riser pipe.
On the other hand, as a fluid mixture including slurry and bubbles injected into the riser pipe at a deep water region or an intermediate-depth water region rises toward a shallow water region, the bubbles increase in volume along with a reduction in the pressure, but the liquid and solid scarcely increase in volume. Accordingly, the volume ratio of bubbles to the fluid mixture increases acceleratingly. As a result, as the fluid mixture approaches the upper end portion of the riser pipe, there arise problems, for example, that the flow velocity of the upward flow becomes excessively high; and a volume ratio of a to-be-lifted substance to the fluid mixture decreases relative to the rest of the mixture, which accordingly lowers the lifting efficiency, or in the worst case, make it impossible to lift the to-be-lifted substance at all. These problems are caused because a relation between the bubble volume and the water depth is a substantially inversely-proportional relation, and become more prominent as the water depth where the to-be-lifted substance exists becomes greater. Meanwhile, in terms of the vertical position inside the rise pipe, these problems occur remarkably when the bubbles rise to a shallow water region.
For example, in the case of gas lift of lifting a to-be-lifted substance from a sea bottom at a water depth of 100 m, the bubbles injected into the lower end portion of the riser pipe increase in volume only 10 times even at the upper end portion of the riser pipe. However, in the case of lifting a to-be-lifted substance from a sea bottom at a water depth of 5,000 m, the bubbles injected into the lower end portion of the riser pipe increase in volume as much as 500 times at the upper end portion of the riser pipe. To be more specific, the volume of bubbles injected at the water depth of 5,000 m increases only by 25% until the bubbles rise to a water depth of 4,000 m, then increases by 5 times at a water depth of 1,000 m, increases by 50 times at a water depth of 100 m, and further increases by 500 times near the water surface.
Here, an estimation is made for a typical case of lifting slurry heavier than seawater under a condition where the flow volume of bubbles is fixedly set, in consideration of erosion or the like, to a volume at which that the flow velocity at the upper end of the riser pipe may not exceed 10 m/sec, for example, even when only seawater is suctioned. In this case, slurry only slightly heavier by several percentages than the surrounding seawater can be lifted if a ratio of the bubbles to the fluid mixture at the upper end inside the riser pipe (hereinafter referred to as a bubble ratio) is controlled at 90% or below.
In addition, in the case of lifting slurry from the sea bottom at a water depth of 1,000 m under the same conditions, it is possible to lift only slurry heavier by at most about 20 percent than the seawater. For this reason, in order to lift a mineral or the like having a high specific gravity, a ratio of seawater to the slurry needs to be kept at a level at which the specific gravity of the slurry is 1.2 or less, whereas the slurry is stuck when a ratio of minerals contained becomes large. However, it is generally difficult to control a mixing ratio in such deep sea.
In summary, the bubble lift in the conventional technique cannot inject bubbles in an amount necessary to lift a substance having higher specific gravity than the surrounding seawater or the like in a deep water region. This is because, as the bubbles injected on the lower side of the riser pipe rise toward a vicinity of the upper end of the riser pipe, the volume of bubbles increases in substantially inverse proportion to the water depth. Hence, the conventional bubble lift has drawback in that the bubble lift does not work at all or is very inefficient even if it works.